Bhubaneswar: In the last 3-decades, the Bay of Bengal had brewed as many as 7 lethal cyclones (including Super Cyclone Amphan) during the pre-Monsoon season of April - May. But the top wind with which 'super' cyclone Amphan pounded the West Bengal coast on May 20 has been among the lowest.
A glance at the data of cyclonic systems formed in the Bay of Bengal (BoB) during the pre-Monsoon season in the last 30-year period has revealed that Amphan is the only Super Cyclone that has made landfall as a very severe cyclonic storm on Wednesday (May 20). And the top wind speed measured stood at 185 km/hr. Only extremely severe cyclonic storm (ESCS) Maha that battered Myanmar in 2006 had the same top gusting wind speed. (See the Table below)
The available data with IMD further reveals that landfall of as many as 6 pre-Monsoon cyclonic systems took place in the month of May. And Super Cyclone Amphan is only the second such cyclonic system to originate in the month of May, especially post the 2nd week of May, when the Madden Julian Oscillation (MJO) remains favourable for high convective activities in BoB. The last such cyclone that saw its genesis post 2nd week of May (14-20) dates back to the year 1997. (Read the table)
While the cyclone season in the Bay of Bengal has been October-November, data shows that November is known as the hottest cyclone month in the BoB. And the month of May has been the most cyclone prone month in the pre-monsoon season.
Data further shows a total of 98 cyclones have made landfall in Odisha during the period of 1891 -2019. And a total of 25 were categorised as cyclones of severe nature (from severe to super cyclone). The ESCS Fani (pronounced as Foni) was the first pre-monsoon cyclone to hit Odisha since 1891.
Why Amphan speed among the lowest?
Since cyclones always originate at a particular latitude near the equator, and as they climb up towards the north, they face rising vertical wind shear that dishevels its uniform centre dense overcast, which leads to rapid weakening.
The super cyclone Amphan has exactly encountered with such hurdles during its north bound journey parallel to Odisha coast. When cyclone Amphan took its birth, the VWS (vertical wind shear) was low (in the range of 10-15 knots), and there it went on to become a super cyclone. But as it climbed up towards Odisha, it faced the hurdle of rising VWS. The VWS rose to around 17 knots, and Super Cyclone Amphan lost its eye.
The top wind speed of cyclone Amphan saw a further drop by around 5 knots before landfall, because, the VWS was around 20-25 knots when Amphan was moving parallel to the Balasore coast and was heading towards Digha and south Parganas coast in WB. A look at the image above clearly shows how the wind speed of Amphan got gradually pruned down as it climbed up towards north.
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